This invention relates to aqueous automatic dishwashing detergent compositions which have a yield value and are shear-thinning which further comprise insoluble inorganic zinc salts, which are useful for inhibiting glassware corrosion in an automatic dishwasher. Compositions of this general type are known. Examples of such compositions are disclosed in U.S. Pat. No. 4,116,851 to Rupe et al, issued Sept. 26, 1978; U.S. Pat. No. 4,431,559 to Ulrich, issued Feb. 14, 1984; U.S. Pat. No. 4,511,487 to Pruhs et al, issued Apr. 16, 1985; U.S. Pat. No. 4,512,908 to Heile, issued Apr. 23, 1985; Canadian Patent No. 1,031,229, Bush et al; European Patent Application No. 0130678, Heile, published Jan. 9, 1985; European Patent Application No. 0176163, Robinson, published Apr. 2, 1986; UK Patent Application No. 2,116,199A, Julemont et al, published Sept. 21, 1983; UK Patent Application No. 2,140,450A, Julemont et al, published Nov. 29, 1984; UK Patent Application No. 2,163,447A, Colarusso, published Feb. 26, 1986; UK Patent Application No. 2,164,350A, Lai et al, published Mar. 19, 1986; U.K. Patent Application No. 2,176,495A, to Drapler et al, published Dec. 31, 1986; and U.K. Patent Application No. 2,185,037A, Dixit, published July 8, 1987.
Corrosion of glass in automatic dishwashers is a well known phenomenon. A paper by D. Joubert and H. Van Daele entitled "Etching of Glassware in Mechanical Dishwashing" in Soap and Chemical Specialties, March, 1971, pp. 62, 64, and 67, discusses the influence of various detergent components, particularly those of an alkaline nature. This subject is also discussed in a paper entitled "The Present Position of Investigations Into the Behavior of Glass During Mechanical Dishwashing" presented by Th. Altenschoepfer in April, 1971, at a symposium in Charleroi, Belgium, on "The Effect of Detergents on Glassware in Domestic Dishwashers". See also, another paper delivered at the same symposium by P. Mayaux entitled "Mechanism of Glass Attack by Chemical Agents".
It has been determined that the glassware corrosion problem actually consists of two separate phenomena; one is corrosion due to the leaching out of minerals from the glass composition itself together with hydrolysis of the silicate network, and the second is deposition and redeposition of silicate material onto the glass. It is a combination of the two that can result in the cloudy appearance of glassware that has been washed repeatedly in an automatic dishwasher. This cloudiness often manifests itself in the early stages as an iridescent film that becomes progressively more opaque with repeated washings.
Use of zinc, in general, in automatic dishwashing to prevent glass corrosion is not new. See for example, U.S. Pat. No. 3,677,820, Rutkowski, issued July 18, 1972, which discloses hanging a strip of metallic zinc in the dishwasher to prevent corrosion of glassware. U.S. Pat. No. 3,255,117, Knapp et al, issued June 7, 1966, discloses the use of soluble zinc salts in automatic dishwashing detergent compositions to prevent glassware corrosion. This reference states that introducing soluble metal salts (alkali aluminate, zincate, or berylliate) in automatic dishwashing detergent compositions can result in precipitation out of insoluble material. Such material is said to be very undesirable as it can adhere to dishwasher parts and dishware during the washing cycle. This precipitation is said to be avoided by carefully adjusting the levels and proportions of the various components in product formulation.
U.S. Pat. No. 3,350,318, Green, issued Oct. 31, 1967, also describes the use of soluble zinc salts (sodium aluminate, sodium zincate) to prevent attack by automatic dishwashing detergent compositions of overglaze colors and decorations on fine china and the aluminum of pots and pans. The problem of precipitate formation is discussed and said to be avoided by spraying a solution of the soluble zinc salt onto granular polyphosphate particles.
U.S. Pat. No. 2,575,576, Bacon et al, issued Nov. 20, 1951, describes the use of a water-soluble zinc or aluminum salt to prevent the corrosion of vitreous and ceramic surfaces. It is stated that the problem of compounding alkali metal salts such as sodium carbonates, -phosphates, -silicates, or -sulfates with water-soluble zinc or aluminum compounds is that an undesirable precipitate is formed. This problem is said to be overcome by the careful choice of particular components at particular ranges and proportions.
U.S. Pat. No. 3,755,180, Austin, issued Aug. 28, 1973, describes use of a precipitated silico-aluminate compound for inhibiting overglaze attack in china. Again, the problem of precipitate formation when soluble zinc and aluminum salts are utilized for this purpose is discussed. (See also U.S. Pat. No. 3,966,627, Gray, issued June 29, 1976.)
Despite these disclosures, there is a continuing need for automatic dishwashing detergent compositions which provide protection against glassware corrosion without causing the formation of insolubles in the dishwasher.
Accordingly, it is an object of the present invention to provide improved liquid automatic dishwashing detergent compositions which provide protection against glassware corrosion without causing the formation of insolubles in the dishwasher which can adhere to dishwasher parts and dishware.
It has been surprisingly discovered that by utilizing certain insoluble inorganic zinc salts in liquid automatic dishwashing compositions, the above objectives can be attained.